Currently the best way to understand reservoir boundaries and their properties is to do a Drillpipe Stem Test. This operation consists of lowering a bottom hole assembly (BHA) with sensors down the wellbore on drill pipe. It is a costly and time consuming operation. With high daily cost drilling rigs (such as but not limited to offshore and deep water rigs) doing a DST is (most of the time) too costly, with the cost being directly associated with the time it takes to do the test. When a DST is imperative to understand complex reservoirs or to estimate reserves, it is often done only in the zones with the greatest chances of success and smaller zones that could still produce a substantial amount of hydrocarbons are neglected as it may be cost prohibitive to perform a DST on these additional zones. Currently the overwhelming majority of DSTs where more than one zone per well are tested are performed by testing one zone per “trip in the hole”, that is only one zone gets tested each time the drillpipe descends into the wellbore.
If information is required from multiple zones of lesser interest, the industry will typically use tools like the Modular Dynamic Tester (MDT), a mark of Schlumberger, tool to assess the zone potential. This method consists of drawing out a small amount of fluid from the formation (usually a few cubic centimeters to a couple of gallons at a maximum) and trying to measure or infer true formation pressure. On one side of the spectrum we have a full DST that involves flowing a substantial amount of formation fluid for extended periods of time to be able to measure or model formation parameters such as pressure, boundaries, skin, permeability, damage, etc. On the other side of the spectrum we have MDT-like measurements where only a comparatively small amount of formation fluid is drawn out and some of the same formation parameters are measured or modeled.
What is needed is a way to use a more efficient system like coiled tubing to deploy the BHA and perform these tests on several zones in one “trip in the hole”. The associated time savings would allow many additional operators to perform a DST on their wells or to increase the number of zones where drillpipe stem testing is being performed. This will increase the knowledge of the reservoir properties, help quantify reserves more accurately and increase the chances of finding more producing zones within the same well.
The idea to use coiled tubing as a means to do a DST test is not new but it faces many safety challenges including but not limited to flowing hydrocarbons to the surface through the coil of the coiled tubing unit. Some of the U.S. Patents or Patent Applications that describe multi-zonal testing systems (and which are incorporated herein by reference) are: U.S. Pat. No. 6,959,763 entitled “Method and apparatus for integrated horizontal selective testing of wells” by Hook and Ramsey; U.S. Pat. No. 6,675,892, entitled “Well Testing Using Multiple Pressure Measurements” by Fikri Kuchuk, et al.; U.S. Pat. No. 7,191,844 entitled “Inflate control system for inflatable straddle stimulation tool” by Kenison et al.; U.S. Patent Application Publication No. 20070044960 titled “Methods, systems and apparatus for coiled tubing testing” by Lovell et al.; and U.S. patent application Ser. No. 11/960,852, filed Dec. 20, 2007, entitled “System and method for optimizing production in a well” by Kannan et al.
Once the formation fluid “exits” the well bore there is only one barrier, the wall of the coiled tubing, to contain the flowing fluid (which may possibly include flammable or toxic hydrocarbons or other materials such as hydrogen sulfide gas). The practice of having only one barrier, especially when that barrier is the coiled tubing itself, is not an approved practice in the industry due to the potential catastrophic consequences of a rupture in the coiled tubing and subsequent release of hydrocarbons or other hazardous materials into the atmosphere. Therefore, although the idea of using coiled tubing to flow a well to surface and to use it as a DST string has been explored in other patents, these patents typically describe unpractical methods (having little or no chance of being used due to safety concerns) or by using what is known in the industry as a dual coil (i.e. concentric coiled tubings) type of equipment.
The present invention proposes that the formation test done in coiled tubing as a solution to the two issues discussed above: the efficiency of a system that can test several zones in a single trip in the hole and a system that will be able to test (draw out) many times over the volume of tools like the MDT tool but without the risk inherent to flowing hydrocarbons to the surface.